In general, various tungsten wires have been widely used as components for discharge electrodes, contact elements, high temperature structural elements, filament material for use in lighting for home electrical appliances or automobile lamps, and cathode heaters for electron guns used in televisions. In particular, tungsten wire, which includes a fixed amount of rhenium (Re), is widely used as a filament material for vibration service lamps and electronic tube heaters, because of its high temperature strength and ductility (shock resistance) after recrystallization.
FIG. 9 is a partial perspective diagram illustrating a component example of a cathode heater 20 which is used in image receiving tubes, and has a construction wherein a tungsten wire (W wire) 21 having a wire diameter of approximately 30 to 50 μm is wound in a spiral as a heating element, with the perimeter thereof coated with an insulation of ceramics film 22. Applying electricity to this cathode heater heats the cathode of the image receiving tube to a high temperature, whereby electrons in the atoms making up the cathode are freed, thereby yielding thermionic discharge.
Tungsten wires for the construction of the above-described cathode heaters and the like, in general have been manufactured using a manufacturing process similar to the description in FIG. 2. That is, a bar of green compact is formed by pressure molding tungsten powder which includes a fixed amount of Re or dopant such as Al, Si, and K, and tungsten sintered compact 1 is prepared by using each end of this green compact as a terminal, passing electricity through and sintering.
Next, after repeating several times the operation of heating the obtained tungsten sintered compact 1 with a heating system 2 for use in swaging and the operation of swaging until the heated sintered compact has a fixed processing rate (working ratio) by using a rotary swaging apparatus (hammering apparatus) 3, the work hardened sintered compact is heated in a heat treating furnace 4 and undergoes a recrystallization process, whereby tungsten wire raw material 1a is obtained. Further, by repeating several times the swaging operation by means of the swaging apparatus 3 and the heating operation by means of the heating system 2 for use in swaging, the processing rate is further increased, and tungsten wire raw material 1b with an even smaller cross-sectional area is formed.
Next, by repeating several times the operation of heating the obtained tungsten wire raw material 1b by means of a wire drawing heating system 5 and the operation of wire drawing the heated tungsten wire raw material 1b to have a fixed wire diameter by means of a wire drawing apparatus 6, a tungsten wire 7 having a predetermined wire diameter was finally manufactured. The manufactured tungsten wire 7 is wound in a form of coil by means of a winding apparatus 8.
However, regarding the tungsten wires manufactured using the above-described conventional manufacturing process, containing for example approximately 3% by mass of rhenium (Re), in a case wherein the wire diameter was 40 μm, after the heating process was completed at a temperature range of approximately 2000 to 2500° C. (equivalent to electricity application heating of 48 to 65% of electricity applied of the fusion current (FC)), the measured value showed the elongation to be 1% or greater. In contrast, however, in a case wherein the heating process was completed at a much higher temperature (for example, a heating process conducted at a temperature above 67% of the FC or higher), the measured value showed the elongation to be 1% or lower. On the other hand, when the wire diameter is large, such as 0.39 mm, the elongation after completing heat treatment for 2 minutes at a temperature range of 1090° C. to 2390° C. was 5% or greater. In other words, tungsten wires with a large diameter yielded sufficient elongation, even when the wires were subjected to high temperatures.
Further, there were no problems and difficulties with parts used at near room temperatures of under 100° C., such as probe pins formed with a conventional W wire having a large diameter.
However, when used under conditions of high temperatures above 1000° C. such as with cathode heaters, or when applied to uses which include a heat treatment process of over 2500° C. during the manufacturing process, there was the problem of easily decreased durability and longevity of the products in use because the strength and elongation would decrease. For example, generally a tungsten wire with a wire diameter 40 μm made of a rhenium-tungsten (Re—W) alloy which includes a predetermined amount of rhenium is used as a component for constituting a cathode heater used in a Braun tube. Further, other Examples of uses where the W wire temperature during use (or during the manufacturing process) reaches 1000° C. or higher, or even exceeds 2500° C., include vibration service lamp filaments used in fields which accompany locomotive movement or vibration, such as do automobiles or pachinko machines. A manufacturing process wherein the W wire temperature exceeds 2500° C. may include flushing operation after coiling and so forth.
As described above, the heat treatment temperature applied to the W raw materials during manufacture of the above cathode heater and so forth is generally a high temperature of 1500° C. or higher, and depending on the situation can exceed 2500° C., and it is desirable for the materials heat-treated at this temperature to possess a large ductility (elongation, stretching), in order to maintain durability and longevity even within this temperature environment. However, thin wires made from Re—W alloy manufactured using conventional manufacturing processes had the difficulties of losing its elongation when heat treated at 2500° C. or higher, or the elongation gradually decreasing as a cathode heater was used for long periods of time, and problems occurred where the heater elements were damaged by minor impact or vibration in the cathode heater and longevity was decreased. Therefore, there is great demand for the development of tungsten wire that possesses excellent durability even when used in conditions of high temperature in this technical field.
Further, regarding the conventional manufacturing methods of tungsten wire, the tungsten wire raw material is prepared by repeating the heat treatment and swaging processing treatment for a predetermined size and length of tungsten sintered compact (sintered body). However, after performing one heat treatment, the processing rate (working ratio) for processing with the swaging apparatus is at most a low value of 10 to 30%. Therefore, in order to process the fixed tungsten thin wire raw material from a tungsten sintered compact, it is necessary to perform numerous repetitions of heat treatment and swaging processing as illustrated in FIG. 2, and while the manufacturing cost of tungsten wire increases due to the increasingly complicated manufacturing process of repetition of heating and swaging, strain (distortion) accumulates and the hardening effects do not work, so that only tungsten wire having a low tensile strength could be obtained.
The present invention has been made to solve the above problems, and it is an object thereof to provide a highly reliable cathode heater and vibration service lamp filament, and to provide tungsten wire which can be manufactured efficiently, and which can exhibit excellent durability when used as a component for cathode heaters, vibration service lamps, and so forth, which are used under conditions of high temperatures or exposed to high temperatures during the manufacturing process.